Installing speakers in my 04 GT?

[Alilconfused]

I took out my 12in sub woofers out of my 04 mustang and I'm attempting to put them in my 04 mustang GT. The GT is an 40th ann also, but my GT has the mach audio system. Which causes a problem getting my speakers in the trunk. The speakers hang down a lil in the trunk and I cant seem to fit my sub woofers in there. Also, the GT has a built in amp that went with package. How can I install my amp and my speakers? I have the 1200watt sony xplods ( speakers & amps) will that be to much on this battery? My other mustang didn't have the audio mach sound system..... I'm clueless.....